1. Field of the Invention
The present invention relates to a rotary valve for a power steering system.
2. Description of the Prior Art
In a conventional rotary valve for a power steering system as shown in FIGS. 1(a) and 1(b), there is provided a valve sleeve 1 in which a rotor 2 is rotatably received. The valve sleeve 1 is formed with supply ports 5 and cylinder ports 6A, 6B therein and serveral axially extending lands on the inner periphery thereof. The rotor 2 is formed with several axially extending lands 3 and 4 alternatively on the outer periphery thereof, wherein the lands 3, provided with exhaust ports 8A and 8B, and the lands 4 cooperate with the lands of the valve sleeve 1 for controlling fluid distribution and fluid pressure to a power cylinder, respectively.
In such a conventional rotary valve, when the rotor 2 is in a neutral position as shown in FIG. 1(a), pressurized fluid supplied from a pump through the supply ports 5 is equally divided and discharged to a reservoir through the exhaust ports 8A and 8B, but when the rotor 2 is rotated, for example, in a counterclockwise direction as shown in FIG. 1(b), the lands 3 are moved to shut off the fluid communication between the supply ports 5 and the exhaust port 8A to thereby increase fluid pressure applied to the cylinder in accordance with the rotational angle .DELTA..theta. relative to the valve sleeve 1. As a result, pressurized fluid is discharged to the exhaust port 8B only through an orifice 7 formed between the land 4 and the valve sleeve 1. Accordingly, the fluid volume passing through the orifice 7 is increased in proportion to the rotation of the rotor 2 relative to the valve sleeve 1. This increase causes the generation of cavitation and unpleasant noise, such as hissing noise. It is known that this disadvantage can be avoided by reducing flow volume passing through the orifice.